This behavior has been observed in 4.18.3, 4.19.2, and Main (CL 4048405). Note that adding BP_Sky_Sphere to the scene and assigning the Directional Light Actor to it will fix the issue.
Sun disc directly visible on Atmospheric Fog does not have correct intensity, resulting in absence of bloom and lens flares. Sun specular reflections in the scene have correct intensity. This energy conservation mismatch leads to very odd looking scenarios.
Result: The Bloom and Lens Flare in the reflection is much more intense than when looking directly at the Sun
Expected: Sun Disc and reflection have same intensity
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-58668 in the post.